"Crimson Tide" vessel, briefly (3)

I believe the answer is:
sub
(Other definitions for sub that I've seen before include "Reserve (abbr)" , "Warship; membership fee" , "Vehicle in water" , "Junior editor" , "Advance payment" .)
I believe the answer is:
sub
(Other definitions for sub that I've seen before include "Reserve (abbr)" , "Warship; membership fee" , "Vehicle in water" , "Junior editor" , "Advance payment" .)